string: Add internal memswap implementation

The prototype is:

  void __memswap (void *restrict p1, void *restrict p2, size_t n)

The function swaps the content of two memory blocks P1 and P2 of
len N.  Memory overlap is NOT handled.

It will be used on qsort optimization.

Checked on x86_64-linux-gnu and aarch64-linux-gnu.
Reviewed-by: Noah Goldstein <goldstein.w.n@gmail.com>